Beginner trying to turn a 'wobbler' crank.

Hi all,

Thought I have a go at trying to make a simple 'wobbler' engine as a beginner project.

I wanted to turn the crank disc and crank pin out of a solid bar to practice off centre turning on something small. My first effort started ok as I did the off centre pin first but when I turned the piece over to face off the reverse I had little material to hold in the chuck and little to measure its setting up in order to get it running true. As you can guess the 2 faces of the crank disc ended up out of parallel.

For a 2nd attempt I have left the crank pin till last but now how can I offset it in the chuck but still have it running true ?

This photo shows my 1st attempt on the left and my 2nd attempt in it's present form on the right.

The top face, sides and centre hole of attempt 2 are all running true to each other (although they don't look it in the photo)

Is there anyway I can now machine the underside to leave a crank pin like the 1st photo ?

I'm clearly not doing the operations in the right order but I'm stuck as to how to proceed and would be grateful for help.

I don't mind scrapping and starting from scratch on a 3rd attempt if someone could tell me how they'd tackle it.

Thanks

Peter.

I may be shot down in flames here as a novice, but I would take the first one and make a simple arbor to hold it.

Get a piece of bar about the same diameter or larger, face off in the lathe, drill and tap a thread for a bolt to hold it in place and drill an offset hole for the crank pin to locate in. Clean it up and use a drop of cyanoacrylate glue to hold it. Face off gently and use a bit of heat to melt the glue afterwards.

Tium's idea would work.

Or you could use the outside jaws on a 3 or 4-jaw chuck so you can push the face of the disk with the pin hard against the inner step. This will make it parallel to the back face and as long as the hole is more or less central it will look OK.

The hard bit is positioning an overhanging tool so you can now face the disk without anything clashing with the chuck jaws - take care.

Neil

Hi Peter

Sorry may be case of the blind trying to lead the blind from me (you know that!) but sounds like you just needed to - from the beginning

Face one side

Reverse in the chuck

face the other side to size

remove from chuck

Mark punch and set to run true your offset diameter (looks like you have enough to hold by)

Turn it

Job Done..

I expect I have it wrong though..

Cheers

Garry

Edited By Gary on 15/06/2015 15:27:21

Edited By Gary on 15/06/2015 15:48:24

Many thanks for the replies

The arbor idea sounds like it would work and seems kinda obvious now you've suggested it !

Hi Garry, I did initially consider what you suggest but thought I'd have difficulty on a small piece (it's about the size of a 10p) When I come to turn the offset I'm then relying on truing it in the chuck based on readings from the faced end rather than its diameter, won't that be inaccurate in a small piece ?

Peter.

Hi Peter

If I'm understanding you correctly once you have marked your offset diameter you will have it in the chuck set with an appropriate parallel against the chuck face then you bring up your tailstock centre to your punch mark to set true. making sure when you have it set that you are firm against the parallel - and of course remove the parallel before you do any turning..

Apologies if I am misunderstanding though Peter..

Hi Garry,

Sorry, I misunderstood what you initially said, I think I see what you mean, what you say makes sense now.

Thanks